Distilled water would be best. Using less antifreeze and more water will decrease the specific gravity (density) of the mix and increase its heat transfer capability. But on the downside as CP points out, you lose freeze protection and lowers the boiling point, so be certain to adjust the mix for the season or drain completely for storage. Better to be safe here. The aluminum cylinder heads transfer more combustion heat into the coolant than the cast iron factory heads did, so there is more heat load into the cooling system that must be transferred to the air via the radiator. Therefore the radiator will need as much help with airflow as possible. Also proper coolant circulation is important. Pulley ratios, water pump impeller clearence etc. should be checked as posted earlier. I had a customer a few years ago that had a similar over heat problem. It turned out that he painted the aluminum radiator so well it insulated it enough and ran hot at low speed. Good luck in finding the cure!
